The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the transmission ratio of the transmission at acceleration.
A known control system for a continuously variable belt-drive transmission has an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a hydraulically shiftable conical disc which is axially shifted by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a line pressure control valve and a transmission ratio control valve. Each valve has a spool to control the oil supplied to the servo devices
The transmission ratio control valve operates to determine the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The output of an engine is transmitted to the drive pulley through a clutch. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
At the start of the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (decrease). The transmission ratio is controlled to a desired transmission ratio.
Japanese Patent Application Laid-Open 60-98251 discloses a control system where a plurality of desired engine speeds Ne' are determined in dependency on a throttle opening degree .theta.. One of the desired speed Ne' is selected at the start and at the end of the acceleration, and the actual engine speed Ne is controlled to coincide with the selected desired speed in dependency on a speed rate e.
However, since the desired engine speed is selected in accordance with engine operating conditions such as the throttle opening degree and whether the vehicle is in a steady state or accelerating, convergence, of the actual engine speed to the desired speed is delayed. In addition, since the desired engine speed is selected from a plurality of stored speeds, the control system becomes complicated.
In order to resolve such problems, there has been proposed a control system where a transmission ratio changing speed (rate) di/dt is calculated in dependency on the difference id-i between the desired transmission ratio id and the transmission ratio i, and on a desired transmission ratio changing speed (rate) did/dt which is for compensating a delay in the hydraulic control operation. A shift quantity D of the spool of the transmission is obtained in accordance with the transmission ratio changing speed (rate) di/dt and the actual ratio i to control the transmission ratio.
Referring to FIG. 5, at acceleration, such as kickdown, the desired transmission ratio id rapidly increases. Accordingly, the actual transmission ratio i approaches the desired ratio id so that the transmission ratio is quickly decreasing. The desired transmission ratio id slowly decreases for decreasing the transmission ratio and the actual transmission ratio i converges toward the desired transmission ratio id while reducing the transmission ratio changing speed (rate). Thus, the transmission ratio is decreased.
However, during such an operation, the acceleration G largely, increases when the transmission ratio changes so as to decrease, which causes shock to the vehicle body.